Engine noise in an internal combustion engine typically is generated by the sudden expansion of combustion chamber exhaust gases. As the combustion gases are exhausted from each cylinder of the engine, a sound wave front travels at rapid sonic velocities through the exhaust system. This wave front is the boundary between the high pressure exhaust pulse and ambient pressure. When the sound wave front exits the exhaust system, it continues to pass through the air until three dimensional diffusion causes it to eventually dissipate. As the wave front passes an object, an overpressure is created at the surface of the object, and it is this overpressure that is a direct cause of audible and objectionable noise.
Since the inception of the internal combustion engine, efforts have been underway to reduce or muffle the noise caused by the engine. Obviously, considerable noise attenuation or reduction can be achieved in a muffler having dimensions that are large enough to permit three dimensional dissipation of the sound waves within the muffler housing. However, from a practical standpoint, design criteria often dictate the size of the muffler which must be kept as small as possible. Further means of reducing engine noise include the use of packing and complex baffle systems. However, these approaches are often accompanied by a substantial increase in the back pressure or resistance of the muffler to the free discharge of the combustion gasses. The increase in backpressure can result in a decrease in the output horsepower of the engine with a resulting loss of efficiency in fuel economy.
Mufflers are classified in various manners within the art. From a structural consideration, mufflers have been classified as being either of two basic types or configurations:                i) a compartmentalized type which comprises several compartments sealed except for the inlets and outlets, the compartments usually being sealed, noise entrapment chambers; or,        ii) a type commonly known as a straight through muffler which usually comprises a duct having a series of perforations within a sealed housing.        
In accordance with this classification, this invention is particularly adaptable to mufflers of the straight through type although, it could have application to compartmentalized type mufflers.
From a functional view, mufflers may be classified as dissipative or reactive. Dissipative mufflers are typically composed of ducts or chambers filled with acoustic absorbing materials such as fibre glass, steel wool or even porous ceramics. Such materials absorb acoustic energy and transform it into thermal energy. Reactive mufflers, on the other hand, are composed of a number of inner connected chambers of various sizes and shapes in which sound waves are reflected to dampen or attenuate waves of a set frequency, typically resonance frequency. This invention relates more to a reactive type muffler.
There are two types of reaction mufflers, a side branch type muffler and a resonator type muffler. A resonator type muffler uses various volumes of different shapes or sizes, i.e., resonance chambers, interconnected with pipes and can dampen not only resonance frequency but also sound waves having frequencies near the resonance frequency. The drawback to resonator mufflers is the large volume required to dampen low frequency sound waves.
The side branch muffler is the type of muffler to which this invention relates. Generally, the side branch muffler has a straight through pipe and an offset or a side branching off the straight through pipe. The side branch is closed at its end and may be bent or shaped with baffles as shown in some of the patents incorporated by reference herein. When the sound wave reaches the closed end of the side branch, it reflects back towards the open end damping waves at the same frequency and out of phase with the reflected wave. The side branch muffler possesses an advantage over the resonator type muffler in that a large volume is not required to dampen any sound wave of a given frequency. However, low frequency sound waves which produce the most objectionable noise require long, side branch lengths which make it difficult to fit within the confines of certain automotive applications.
Apart from the functional and structural discussion above, sports cars and high performance vehicles have additional requirements. It has long been known that the exhaust systems of such vehicles must be tuned to emit certain sounds from the automobile which appeal to the purchaser of such vehicles while satisfying noise regulations. Such applications require attenuation of specific waves having set frequencies to produce the desired sound. More particularly, high performance mufflers of the type under discussion are tuned to the specific type of engine to which the muffler will be applied to. Specifically, the valving or breathing characteristics of the engine are matched to the muffler over the operating range of the engine to produce the desired tone. Recent engineering advances in the structural rigidity of the body or chassis of the vehicle in which the engine is mounted have enhanced the sound of the engine within the cabin of the vehicle. Specifically, a muffler could be tuned to a desired sound with the engine on a test stand, but produce objectionable resonance in the cabin. Since the cabin cannot be dampened, the muffler has to be precisely tuned to attenuate the sound waves producing the objectionable resonance within the cabin.
The side branch type muffler, in theory, has the ability to resolve this problem. However, the approach followed was random and haphazard and simply involved reconstructing entirely different side branch designs until one resulted in the removal of the objectionable noise. Unfortunately, the length of the side branch typically exceeded the space limitations for the muffler design.
The Huff patents above (U.S. Pat. Nos. 6,595,319; 6,199,658; and 5,952,625) overcame many of these problems with a side branch type muffler which can be readily tuned to produce any desired sound in a compact design avoiding the space limitations afflicting conventional side type mufflers. In this respect, the Huff patents show a muffler with an inner cylindrical casing axially extending from the inlet through the outlet and defining an open ended inner chamber contained therein through which the exhaust gases pass. An outer concentric casing with axial end sections is spaced radially outward from the inner casing and defines therebetween a closed end outer chamber. A slotted opening arrangement at a set axial position provides fluid communication between the inner and outer chamber. A sound attenuating arrangement within the outer chamber includes a plurality of intermediate, cylindrical casings which axially extend substantially the length of the outer chamber and are radially spaced to overlie one another so that each pair of radially adjacent casings forms an annular, axially extending sound attenuation passage. Each sound passage has an entrance in fluid communication with a pressure wave at one end thereof and a sound reflection wall at its opposite end to establish a second path therebetween. Certain select sound passages have an entrance in fluid communication with the slotted opening while other sound passages have an entrance in fluid communication with an adjacent sound passage whereby a plurality of sound passages having various sound path lengths is produced for reflecting and attenuating a plurality of sound waves at set frequencies, particularly sound waves of low frequency. It was found that this muffler configuration is effective in eliminating objectionable sounds.
Further, the Huff muffler can be modified to include at least one annular stop plate extending within a selected sound passage between radially adjacent intermediate casings forming the selected sound passage. The stop plate is positioned at a set axial distance within the selected sound passage correlated to the axial distance a sound wave travels from a passage entrance to the stop plate whereby any sound wave of any specific frequency may be attenuated by positioning the stop plate at a set axial distance in a sound passage thus permitting the muffler to be tuned to any desired sound.
However, while the Huff muffler is effective, it is limited in its application due to the size of the radially spaced sound passages. In this respect, the radially spaced sound passage produce a muffler that is cylindrical with a side dimension equal to the height dimension. Many applications have different height and width requirement wherein the cylindrical configuration exceeds one of these dimensional limitations. Further, additional sound chambers, to attenuate multiple frequencies, increase the length of the muffler wherein it can be too long for certain vehicles. The Huff mufflers also can be costly to manufacture in that they require successively decreasing radial height for each successively larger diameter sound passages of the outer passages to avoid pressure undulations and accompanying sound wave variations as the waves travel in a sound path from one sound passage to another radially spaced sound passage.